The present invention relates to an image forming apparatus capable of solving the problem of a backside soil produced on a transfer material when a toner image is transferred from a photoreceptor or an intermediate transfer member onto the transfer material.
In a conventional color image forming apparatus without depending on the intermediate transfer method or transfer material direct transfer method, each photoreceptor is kept in contact with a conveyance belt for conveying an intermediate transfer member or a transfer material. While an endless belt-like intermediate transfer member as the intermediate transfer member or a conveyance belt is in contact with the photoreceptor, each photoreceptor or developing device cannot be stopped independently. If either the photoreceptor or the belt is stopped while they are in contact with each other, the rotating member is damaged by rubbing. To prevent this damage, means are provided in such a way that each drive is stopped after the photoreceptor has been separated completely from the endless belt-like intermediate transfer member or the conveyance belt. Further, when the developing device is stopped independently of the photoreceptor, toner or carrier may deposit on the photoreceptor, depending on the potential of the photoreceptor. To prevent this, means are provided to ensure that, after the photoreceptor can be stopped, a charging device is stopped, and then the photoreceptor and the developing device are stopped. Since the developing function is constantly working, toner is transferred also in the non-image area between the image areas, and a certain amount of image fog may be transferred in some cases.
There is no problem when a secondary transfer roller for transferring from the intermediate transfer member onto paper as a transfer material P is provided with a cleaning device such as a blade or brush, because cleaning is carried out at all times. However, when there is no cleaning device to clean the secondary transfer roller, toner deposited on the non-image area between an image area and the next one is attached to the secondary transfer roller. This is transferred onto the back surface of the next sheet of paper to cause a backside soil of paper. Conversely, when a secondary transfer roller is provided, a different problem arises. To be more specific, when the blade or brush is brought into contact, the toner must be scraped off by one cleaning operation. This requires the pressure to be increased, and the speed difference to be applied in the case of brush. However, this arrangement increases the torque of the secondary transfer roller and may require use of an exclusive driving apparatus. Thus, there has been a demand for a device of stable and simple structure, capable of solving the problem of contamination of paper by toner, without requiring use of an exclusive driving apparatus.
The Patent Document 1 discloses a transfer process, wherein transfer is carried out by contact electrification without a cleaning device, and the amount of electric charge applied to a photoreceptor is changed and inputted in response to the output of an environmental status detector. Measures are taken to remove a fog in conformity to a particular environmental status, according to this disclosure. However, this Patent Document 1 is not sufficient to meet the requirements.    (Patent Document 1): Official Gazette of Japanese Patent Tokkai 2001-350323.